Aluminum heat exchangers, radiators, evaporators, condensers and the like, are typically manufactured from aluminum alloy that is coated with a thin surface layer of braze material, which has a lower melting point than the base alloy layer. The various components of the heat exchanger, tanks, slotted header plates, tubes, cooling fins, etc, are held together temporarily with various fixtures, and then cycled through a braze oven. In the braze oven, the heat exchanger is brought to a temperature just sufficient to melt the surface layer of braze material, but not the base alloy. The parts are held together with a close clearance wherever a braze joint is desired, as between tube ends and tank header slots, or between cooling fin peaks and tube surfaces. In these close clearances, molten braze material is drawn in by capillary action, later solidifying to form braze joints. The braze process per se is proceeded by a fluxing step, in which the part is sprayed with a coating of waterborne chemical that acts to remove any oxidation on the part during the braze process. The part must be dried, leaving only the solid flux layer, before brazing. The flux chemical acts to absorb any surface oxidation that would otherwise interfere with the formation of good braze seams.
Braze ovens include the radiant heat type, in which a part is conveyed through a space that has heat simply radiated passively into it, as well as newer, convection types, such as that shown in coassigned U.S. Pat. No. 5,172,847. Convection type furnaces actively force heated gas, which must be an inert gas such as nitrogen, at the part, so as to heat it more quickly. The part must be brought to braze temperature, that is, to the temperature at which the braze layer liquifies, quickly enough so that the part will have time enough in the braze cycle to form the necessary braze seams and joints. One relatively new type of brazed heat exchanger is the so called tank and tube or headered condenser, an example of which may be seen in coassigned U.S. Pat. No. 5,062,476. This design has a pair of side tanks that are extruded from a fairly thick aluminum stock, so as to resist the relatively high pressures involved. As a consequence, the sides of the condenser are high in mass, and a concentrated mass, with relatively little exposed surface area per unit of mass. The tubes and fins that run between the tanks, however, are lower in mass, since they must conduct heat quickly, with a lot of exposed surface area. Consequently, there is a significant potential time delay between the high and low mass areas of the part reaching braze temperature, within a given braze cycle time, which might necessitate the use of longer braze cycle times to let the higher, more concentrated mass areas "catch up". This means longer, larger, and consequently more expensive braze ovens.